Because of the large inherent bandwidth of optical systems, such systems are being increasingly utilized for transmitting information, generally over fiber optic cables, in telephony, cable television, data processing and other applications. Heretofore, such systems have typically used amplitude modulation because it is simple to implement. However, frequency modulation can provide an order of magnitude or more increase in signal-to-noise ratio. This is because the predominant noise sources are sources of amplitude noise rather than frequency noise. While this signal-to-noise ratio advantage has been taken advantage of for many years in the radio field, FM is not currently being employed to any extent in optical communications for a number of reasons.
One reason why optical FM communications are not more extensively utilized is that current applications are typically binary. For such applications, the signal-to-noise ratio improvement provided by FM is in most applications not sufficiently important to warrant the increased cost and complexity of an FM system. For analog communication or communication involving a multiplicity of discrete optical frequency levels, where noise may present problems, current optical frequency discriminators have been found to provide linearity over such a short frequency range that it is difficult to obtain distortion free outputs over a useful range of modulating frequencies.
The lack of an optical frequency discriminator which provides a substantially linear output over an extended frequency range is also a problem in other applications, for example, in many applications where accurate, high speed testing or measurement of the optical frequency of a changing signal is required. Frequency shift key (FSK) systems are one such application.
A need therefore exists for an improved frequency discriminator for use in optical FM communication systems and other applications, which discriminator provides a substantially linear output over an enlarged frequency range. It is an object of this invention to provide such a discriminator. A further object is to provide an enhanced optical FM communications system capable of providing distortion-free operation over an extended frequency range.